In the field of geophysical prospecting, the knowledge of the subsurface structure of the earth is useful for finding and extracting valuable mineral resources such as oil and natural gas. A well-known tool of geophysical prospecting is a “seismic survey.” In a seismic survey, acoustic waves produced by one or more sources are transmitted into the earth as an acoustic signal. When the acoustic signal encounters an interface between two subsurface strata having different acoustic impedances, a portion of the acoustic signal is reflected back to the earth's surface. Sensors detect these reflected portions of the acoustic signal, and outputs of the sensors are recorded as data. Seismic data processing techniques are then applied to the collected data to estimate the subsurface structure. Such surveys can be performed on land or in water.
In a typical marine seismic survey, multiple streamer cables are towed behind a vessel. A typical streamer includes multiple seismic sensors positioned at spaced intervals along its length. Several streamers are often positioned in parallel over a survey region. One or more seismic sources (such as air guns or marine vibrators) are also normally towed behind the vessel.
The signals received by sensors in marine streamers are contaminated with noise to varying degrees. This noise typically has many different origins. One major source of noise is “tow noise” resulting from pressure fluctuations and vibrations created as the streamer is pulled through the water by the vessel.
Currently, one of the main techniques used to reduce tow noise involves grouping adjacent sensors and hard-wiring the outputs of the sensors in each group together to sum their respective analog output signals. A typical sensor group contains eight to sixteen spaced apart sensors. Each group may span between 10 and 20 meters. Since the individual sensors in each group are fairly closely spaced, it is assumed that all the sensors in a given group receive substantially the same seismic signal. The seismic signal is therefore reinforced by the summing of the analog output signals of the hydrophones of the group and the particle motion sensors of their corresponding group. Random and uncorrelated noise affecting each sensor, on the other hand, tends to be cancelled out by the summing process. The gain of eight to sixteen over the output of an individual sensor provides quite good rejection of random noise.
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